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<H2>Widgets</H2>

<P>SWT includes many rich features, but a basic knowledge of the system's core -
<em>widgets</em>, <em>layouts</em>, and <em>events</em> - is all that is needed
to implement useful and robust applications.</P>


<H3>Widget application structure</H3>

<P>When you are contributing UI elements using platform workbench extensions,
the mechanics of starting up SWT are handled for you by the workbench.</P>

<P>If you are writing an SWT application from scratch outside of the
workbench, you must understand more about SWT's application structure.</P>

<P>A typical stand-alone SWT application has the following structure:</P>

<ul>
  <li>Create a
      <strong><a href="../reference/api/org/eclipse/swt/widgets/Display.html">Display</a></strong>
      which represents an SWT session.</li>
  <li>Create one or more
      <strong><a href="../reference/api/org/eclipse/swt/widgets/Shell.html"> Shell</a></strong>s
      which serve as the main window(s) for the application.</li>
  <li>Create any other widgets that are needed inside the shell.</li>
  <li>Initialize the sizes and other necessary state for the widgets. Register
      listeners for widget events that need to be handled.</li>
  <li>Open the shell window.</li>
  <li>Run the event dispatching loop until an exit condition occurs, which
      is typically when the main shell window is closed by the user.</li>
  <li>Dispose the display.</li>
</ul>

<P>The following code snippet is adapted from the
<strong><a href="../samples/org.eclipse.swt.examples/doc-html/swt_helloworld_ex.html">org.eclipse.swt.examples.helloworld.HelloWorld2</a></strong>
application. Since the application only displays the string &quot;Hello World,&quot;
it does not need to register for any widget events.</P>

<pre>
   public static void main (String [] args) {
      Display display = new Display ();
      Shell shell = new Shell (display);
      Label label = new Label (shell, SWT.CENTER);
      label.setText (&quot;Hello_world&quot;);
      label.setBounds (shell.getClientArea ());
      shell.open ();
      while (!shell.isDisposed ()) {
         if (!display.readAndDispatch ()) display.sleep ();
      }
      display.dispose ();
   }
</pre>


<H4>Display</H4>

<P>The <strong><a href="../reference/api/org/eclipse/swt/widgets/Display.html">Display</a></strong>
represents the connection between SWT and the underlying platform's GUI system.
Displays are primarily used to manage the platform event loop and control
communication between the UI thread and other threads. (See
<a HREF="swt_threading.htm" CLASS="XRef">Threading issues for clients</a> for
a complete discussion of UI threading issues.)</P>

<P>For most applications you can follow the pattern that is used above.  You
must create a display before creating any windows, and you must dispose of
the display when your shell is closed. You don't need to think about the
display much more unless you are designing a multi-threaded application.</P>


<H4>Shell</H4>

<P>A <strong><a href="../reference/api/org/eclipse/swt/widgets/Shell.html">Shell</a></strong>
is a &quot;window&quot; managed by the OS platform window manager. Top level
shells are those that are created as a child of the display. These windows are
the windows that users move, resize, minimize, and maximize while using the
application. Secondary shells are those that are created as a child of another
shell. These windows are typically used as dialog windows or other transient
windows that only exist in the context of another window.</P>


<H4>Parents and children</H4>

<P>All widgets that are not top level shells must have a parent. Top level
shells do not have a parent, but they are created in association with a
particular
<strong><a href="../reference/api/org/eclipse/swt/widgets/Display.html">Display</a></strong>.
You can access this display using <strong>getDisplay()</strong>.
All other widgets are created as descendants (direct or indirect) of top level
shells.</P>

<P><strong><a href="../reference/api/org/eclipse/swt/widgets/Composite.html">Composite</a></strong>
widgets are widgets that can have children.</P>

<P>When you see an application window, you can think of it as a widget tree, or
hierarchy, whose root is the shell. Depending on the complexity of the
application, there may be a single child of the shell, several children, or
nested layers of composites with children.</P>


<H4>Style bits</H4>

<P>Some widget properties must be set at the time a widget is created and
cannot be subsequently changed. For example, a list may be single or
multi-selection, and may or may not have scroll bars.</P>

<P>These properties, called <em>styles</em>, are set in the constructor.  All
widget constructors take an <strong>int</strong> argument that specifies
the bitwise <strong>OR</strong> of all desired styles. In some cases, a
particular style is considered a hint, which means that it may not be
available on all platforms, but will be gracefully ignored on platforms that
do not support it.</P>

<P>The style constants are located in the
<strong><a href="../reference/api/org/eclipse/swt/SWT.html">SWT</a></strong>
class as public static fields. A list of applicable constants for each
widget class is contained in the API Reference for
<strong><a href="../reference/api/org/eclipse/swt/SWT.html">SWT</a></strong>.</P>


<H4>Resource disposal</H4>

<P>The platforms underneath SWT require explicit allocation and freeing of
OS resources.  In keeping with the SWT design philosophy of reflecting the
platform application structure in the widget toolkit, SWT requires that you
explicitly free any OS resources that you have allocated.  In SWT, the
<strong>Widget.dispose()</strong> method is used to free resources associated
with a particular toolkit object.</P>

<P>The rule of thumb is that if you create the object, you must dispose of it.
Here are some specific ground rules that further explain this philosophy:</P>

<ul>
  <li>If you create a graphic object or widget using a constructor, you must
      explicitely dispose of it when you are finished using it.</li>
  <li>When a
      <a href="../reference/api/org/eclipse/swt/widgets/Composite.html"><strong>Composite</strong></a>,
      is disposed, the composite and all of its child widgets are recursively disposed. In this
      case, you do not need to dispose of the widgets themselves. However, you
      must free any graphics resources allocated in conjunction with those widgets.</li>
  <li>If you get a graphic object or widget without using a constructor (e.g. Control.getBackground()),
      do not dispose of it since you did not allocate it.</li>
  <li>If you pass a reference to your widget or graphic object to another
      object, you must take care not to dispose of it while it is still
      being used.  (Similar to the rule described in
      <a HREF="jface_resources.htm#jface_resources_patterns" CLASS="XRef">Plug-in
      patterns for using images</a>.)</li>
  <li>If you create a graphic object for use during the lifetime of one of your
      widgets, you must dispose of the graphic object when the widget is
      disposed. This can be done by registering a dispose listener for your
      widget and freeing the graphic object when the <strong>dispose</strong>
      event is received.</li>
</ul>

<p>There is one exception to these rules.  Simple data objects, such as
<strong><a href="../reference/api/org/eclipse/swt/graphics/Rectangle.html">Rectangle</a></strong> and
<strong><a href="../reference/api/org/eclipse/swt/graphics/Point.html">Point</a></strong>,
do not use operating system resources. They do not have a
<strong>dispose()</strong> method and you do not have to free them.  If in
doubt, check the javadoc for a particular class.</p>

<p>See <a href="http://www.eclipse.org/articles/swt-design-2/swt-design-2.html">Managing
operating resources</a> for further discussion of this topic.</p>

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